The present invention relates to a termination for an electrical circuit with improved electrical and structural integrity.
Electrical connections between and amongst electrical components include the use of flexible cables and circuits whose termination areas, including plated-through holes and other interconnections with the components, are often made more rigid relative to the other portions of the cables or circuits. Such flexible printed wiring includes layers of electrical insulation and conductive leads bonded thereto. For military, aerospace and other high technology applications the electrical insulation typically comprises a polyimide film identified, for example, by the trademark Kapton, although other dielectric films, such as fluorinated ethylene polymer (FEP), are used. A polyimide is generally preferred (although FEP has a slightly higher service temperature than polyimide e.g., between -200.degree. C. to 300.degree. C.), because of its higher tensile strength, lower thermal coefficient of expansion and higher dielectric strength.
Also, even though FEP can be bonded to itself while a polyimide has no adhesive properties of its own, a polyimide is still preferred. Accordingly, a polymide must be coated with an adhesive, which typically comprises an acrylic based compound. The metallic leads, usually formed from copper foil, are also adhesively bonded to the dielectric film by the acrylic based compound.
Thus, a conventional flexible printed circuit may comprise a centrally positioned copper-clad Kapton sheet which is sandwiched between two Kapton coversheets, sometimes termed "coverlays." Bonding is by an acrylic adhesive. Depending upon electrical interconnection requirements, generally a plurality of such sandwiched flexible printed circuits are combined in which the copper cladding comprise combinations of signal, power and shield conductors.
Flexible printed circuits are terminated by bonding one or more such layers together along with rigidizing members, and the whole becomes an integral bonded-together unit. A typical rigidizing member comprises layers of copper clad polyimide glass placed on the external surfaces of the printed circuitry, and the whole is bonded together by an acrylic adhesive. Thus, a conventional termination includes layers of flexible printed circuitry whose termini are bonded together with acrylic adhesive and layers of polyimide glass bonded to the exposed interfaces of the circuitry also with acrylic adhesive. Plated-through holes interconnect the conductors in the several layers, and electric components are soldered to and within the plated-through holes.
It is important in such constructions that the pads on the external copper clad polyimide glass rigidizig members and all of the conductors of the respective layers of flexible printed circuitry be aligned to ensure that electrical connections amongst the layers of circuitry in fact exist. The requirement, that these layers be accurately aligned so that the plated through holes can make effective electrical connections, has dictated the use of extremely accurate and sophisticated registration tooling. The almost universal use of precision plotters for multi layer artwork generation and of numerically controlled drills for multi layer board drilling is an outgrowth of these requirements. The outer layer circuit patterns are plated with a tin-lead alloy, and then fused to provide a readily solderable surface with good storage characteristics. It is preferred that the outer layers contain only pads or lands and no conductors.
One major difficulty encountered with such terminations of flexible printed wiring combinations lies with the use of acrylic adhesive. As distinguished from polyimide materials, which are thermosetting, acrylic adhesives do not cure but remain thermoplastic; therefore, they remain rubbery and are not rigid. However, acrylic adhesives are used as distinguished from other materials because they are flexible, have good dielectric characteristics, are resistant to solvents, do not flow under pressure, and remain stable between -50.degree. C. through 215.degree. C., which are the minimum temperature requirements for military and some aerospace applications. The disadvantages of acrylic adhesives are due to their thermoplastic properties.
Specifically, because acrylic adhesives do not set, they provide a medium by which the lands of the polyimide layers they join might move horizontally with two degrees of freedom in an x-y direction. Such movement produces vertical misalignment among the conductors and pads in the several layers, and results in poor or no interconnections. Further, acrylic tends to expand with a rise in temperature, which increases the thickness of the joined together layers, and exerts tension on the plated through holes which separate and break the vertical or "z axis" interconnections.
A further problem is that acrylic adhesives as well as the polyimide layers smear when holes are drilled through the terminations prior to being plated with copper and this smear must be removed prior to the plating. When the holes are drilled through the assembly to expose copper of the conductors at all points where lands are to be located, whether internal or external, the drilling smears the otherwise exposed copper with the plastic layers and adhesives, especially when the temperature of the drill bit at the cutting interface in the hole rises above the glass transition temperature (T.sub.5) or the plastic temperature range of the plastic and thus leaves an insulating layer on the drilled through copper conductors. Such smear is removed before the plating process by an acid solution or, preferably, by a plasma etch. Typically, the circuit is first baked for approximately 3-8 hours, and followed by a plasma etching for about 20 minutes.
One further problem with conventional termination process is the need to place a circuit within a pouch when the termination is chemically etched. This "pouching" protects already completed portions from being etched, and must be carefully performed.
Still another problem, related to the flexibility of the cable, is the use of solid conductors for shielding purposes. Such conductors typically have a 1.4 mil (3.56 microns) thickness and as a result are not sufficiently flexible.